There are numerous processes for trimerizing organic polyisocyanates (J. H. Saunders and K. C. Frisch, Polyurethanes: Chemistry and Technology, pages 94 et seq (1962)). In addition to strongly basic compounds, such as the alkaline-acting metal salts of organic acids and tertiary amines, the catalysts used for the trimerizing reaction are primarily tertiary phosphines of the aliphatic and mixed aliphatic-aromatic series.
In cases where polymerization is continued to completion with development of a plurality of isocyanurate rings, insoluble end products are obtained. In cases where polymerization is prematurely terminated, for example, by deactivating the catalyst, polymerization products having stillsoluble NCO-groups, an isocyanurate structure and a higher functionality by comparison with the starting isocyanates are obtained. By virtue of their polyfunctionality and the fast crosslinking with hydroxyl group-containing resins which this facilitates, these relatively high molecular weight soluble polyisocyanates of isocyanurate structure are used in polyurethane chemistry, for example, as crosslinkers, in making lacquers and adhesives. One factor of considerable commercial interest is that polyisocyanates of this type still contain a useful number of NCO-groups. In spite of this, the polyisocyanates contain a very small proportion of monomeric diisocyanates. This is particularly important because extensive toxicological investigations and several years experience in the processing of these products have shown that for these polyisocyanates an upper limit of monomer content of 0.7%, based on the solids content, guarantees safe processing of lacquers produced from them, providing the safety measures normally taken in the processing of lacquers are observed. The above-mentioned limit of 0.7% has been adopted in the literature, for example, in the pamphlet entitled "PUR-Anstrichstoffe" issued by the Hauptverband der deutschen gewerblichen Berufgenossenschaft (1971), and in the Paintmakers Associations's "Polyurethane Report", December, 1970.
According to German Pat. No. 1,201,992, relatively high molecular weight polyisocyanates having an isocyanurate structure and a monomer content of less than 1.0%, based on the solids content, may be obtained by reacting 3,3'-diisocyanato-4,4'-dimethyl diphenyl uretdione with catalytic quantities of tertiary phosphines, preferably aliphatic tertiary phosphines, at temperatures below 100.degree. C. in solvents in which the uretdione is sparingly soluble and the reaction product readily soluble. Conversion of the uretdione ring into the cyanurate structure takes place with partial splitting of the uretdione ring, this splitting reaction representing a temperature-dependent reversible equilibrium reaction in accordance with the following scheme: ##STR1##
In addition, British Pat. No. 949,253 describes a process in which the trimerization of tolylene diisocyanate is carried out at temperatures of from 50.degree. to 70.degree. C. in the presence of basic catalysts, preferably metal salts of organic acids, in combination with mono-N-substituted carbamic acid esters as co-catalysts. The polyisocyanates of isocyanurate structure obtained by this process have NCO-contents of from 13 to 15% and monomer contents of less than 1.25%, based on solids.
Although catalysis with phosphines in accordance with German Pat. No. 1,201,992 and the process according to British Pat. No. 949,253 allow the production of substantially colorless polymers having a low monomer content below 1.25% to be obtained at temperatures in the range of from 50.degree. to 70.degree. C., they are nevertheless attended by the following disadvantages:
(1) Phosphines in general and aliphatic phosphines in particular are highly sensitive compounds which may ignite simply on contact with air. In addition, phosphines have an extremely unpleasant, intoxicating odor and are highly toxic. Accordingly, from the point of view of safety and also from the physiological point of view, phosphine catalysts are extremely sensitive compounds which have to be handled with caution.
(2) In these processes, the polymerization reaction using phosphine catalysts lasts between 9 and 11 days, which is uneconomical because of the poor volume-time yield.
(3) The required specification of 0.7% free tolylene diisocyanate may only be achieved by using certain solvents, for example, butyl acetate. In contrast, it cannot be achieved using, for example, ethyl acetate and in such case the monomer content has to be subsequently lowered by the addition of monohydric alcohols, as described in German Offenlegungsschrift 2,414,413.
(4) In cases where 2,4-tolylene diisocyanate or mixtures containing 2,4-tolylene diisocyanate as their main component are used, large quantities of solid reaction products are formed with phosphine catalysts at temperatures below 100.degree. C. These solids consist partly of 3,3'-diisocyanato-4,4'-dimethyl diphenyl uretdione and partly of addition compounds of isocyanurate isocyanates with excess tolylene diisocyanate. As a thick crystal sludge which is virtually impossible to stir, these reaction products complicate handling of the reaction mixtures and also heat transfer to a very considerable extent.
(5) With aromatic polyisocyanates in which the two isocyanate groups are equal in reactivity, the treatment with tertiary phosphines as catalysts, in the presence or absence of solvents, gives rise to the formation of high molecular weight polymers of uretdione structures. These uretdiones have poor solubility and are therefore unsuitable for further processing as reaction lacquers or as a component of adhesives.
(6) According to British Pat. No. 949,253, a mono-N-substituted carbamic acid ester is used as co-catalyst in addition to the basic catalyst for the production of low-monomer polyisocyanates of isocyanate structure from tolylene diisocyanate. In addition, the NCO-content of the polymer has to be reduced to between 15 and 13% in order to be able to guarantee a monomer content of 1.0%, based on solids.